- Email: [email protected]
Carbon to chlorophyll-a ratio in modeling long-term eutrophication phenomena
~
Pergamon
Wat. ScL Tech. Vol. 38. No. II. pp. 227-23.5.1998. ~ 1998IAWQ .
.
PIT: S0273-1223(98)00659-3
. Published by Elsevier Science LId
Printed In Great Britain. All rights reserved 0273-1223198 $19'00 + 0-00
CARBON TO CHLOROPHYLL-A RATIO IN MODELING LONG-TERM EUTROPHICATION PHENOMENA Haisheng lin *, Shinji Egashira** and K. W. Chau *** • R&D Center. Nippon Koei Co.• Ltd. 2304 Takasaki. Kukizaki-cho. lnashiki-gun • Ibaraki 300. Japan •• Faculty a/Science and Engineering. Ritsumeikan University. ]-]-] Noji-higashi. Kusatsu. Shiga 525. Japan ••• Department a/Civil and Structural Engineering. Hong Kong Polytechnic University. Hung Hom. Kowloon. Hong Kong
ABSTRACT Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur 50 as to max~ize the specifi.c growth rate for ambient conditions. including solar radiation and water temperature. With the dynamiC CCHL. an unsteady two-layered. two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (1s) is determined as weighted average of the light i.ntensit~ ~or previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxIc condition during summer in Tolo Harbour. Hong Kong is successfully reproduced by the present method. Otherwise. the simulation with a constant CCHL gave a wrong result. 1998 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Carbon to chlorophyll-a ratio; density stratification; dissolved oxygen; eutrophication; phytoplankton; tidal flow.
INTRODUCTION Eutrophication. due to excessive grow~ of aquatic plants ~o levels resulting in the interference with desirable water uses. is one of the most Important water quality problems. The polluting material yielded from the extensive industrial West European basins of Rhine. Meuse and Scheidt has inevitably caused a serious pollution in the North Sea (Ede~ •. 19~3). Nutrie.nts in kraft pulp mill effluent have resulted in major changes in the level of algal produc~vlty In som~ nve~ of western North ~merica (Bothwell. 1992). Chesapeake Bay in USA is plagued WI~ probl~ms. including bottom-water. anoxIa, decline in fisheries and so on. that accompany agricultural and Industnal developme~t an~ population growth along its shores and headwaters (Cerco and Cole. 1993). Since 1977. algal bloom In Blwa lake of Japan has usually occurred in early summer (SWEC. 1996). In Hong Kong. most o~ the. coastal water~ recei~e sewage discharges directly from the urbanized catchments. as well as domestic. livestock and Industnal sources via streams and stormwater runoffs. which results in excessive nutrient contents with high eutrophic potential and development of algal blooms including red tides. In Tolo Harbour of Hong Kong. there were 89 red tide 227
228
H.lIN elal.
occurrences with 39 cases recorded in 1988. The monitored maximum chlorophyll-a concentrations were above 80mgll in 1986, 70mgll in 1988 and 60mgll in 1989, respectively (EPDHK, 1990). In general, mean chlorophyll-a levels above IOmgll can be regarded as unacceptably high and such levels indicate eutrophication. For natural offshore oceanic waters, the chlorophyll-a levels are below 2mgll. Tolo waters can be regarded as highly eutrophic. The problem has been attended seriously. Much effort and many countermeasures have been conducted in recovering the equilibrium of ecosystem (Thomann and Mueller, 1987; Ambrose et al., 1988; Lee et al., 1991; Cerco and Cole, 1993). With the aim of predicting long-term water quality transport and fate accompanied by eutrophication in a density stratified natural waterbody, a two-layered, two-dimensional eutrophication numerical model has been developed and calibrated by the scenario in Tolo Harbour of Hong Kong. The model is based on a numerical-generated boundary-fitted orthogonal curvilinear co-ordinate system. The hydrodynamic parameters are predicted simultaneously (Chau and lin, 1995). Pollution sources are locally specified. Referring to in situ sampling analysis, the sediment oxygen demand and nutrients released from sediment are incorporated in the modeling. In this paper, an important parameter CCHL Carbon to CHLorophyll-a ratio is emphasized and corresponding results are discussed. BRIEF DESCRIPTION OF EUTROPHICATION MODELING
In a water quality system, density stratification usually forms, which inhibits vertical transport and suppresses the connection between benthic grazers and near-surface biomass (Salhotra et al., 1987). Despite dissolved oxygen content in most of the surface waters are at satisfactory levels, even at super-saturation, serious oxygen depletion, approaching anoxic conditions may occur in the lower water (Cerco and Cole, 1993).
Tolo Harbour is a nearly land-locked sea inlet with water surface area of about 52 km 2 as shown in Figure I. There are approximate 16km long from southwest at inner harbour to northeast at outer channel. The water depth is from about 2m in the inner part to over 20m in the outer and about 12m on average. For most of the year, little freshwater flows into the harbour. During summer, the differences of temperature as well as salinity in surface and bottom water results in density stratification, which exhibits an obvious lighter surface layer and a definite mesolimnion. In winter, the stratification almost diminishes. Multi-yearly-averaged daily density variation in both the surface and the bottom water is shown in Figure 2(a). It is much better to describe the transport and fate of water quality with a two-layer-averaged system in the vertical water column instead of a common depth-averaged method (Yih Chia-Shun, 1980). According to the measured data, the layer interface position is located at Zo=-6m (M.C.D. - Metre Chart Datum) for the Tolo waters. Tolo Harbour is located in subtropical zone. Figure 2(b) shows its daily solar radiation intensity during 1985-1989. The New Territories, Hong Kong KauLauWan
o
• monitoring station
I 2km
C'"n-e-H-On-g-K-O-ns-u-I-an-d-)
Figure 1. Tolo Harbour, Hong Kong.
Pollution sources: - main stream a outfall
Carbon 10 chlorophyll-a ratio
229
(a) Water density Figure 2. Water density and daily solar radiation in Tolo Harbour. Hong Kong.
Due to irregular computational domain, a numerically generated boundary-fitted orthogonal curvilinear co• ordinate system, as shown in Figure 3, is employed. In orthogonal curvilinear coordinates, the present method numerically solves partial differential equations describing conservation of mass and momentum of incompressible fluid as well as transport of po.llutants over the depth of each layer or over the total depth in areas too shallow for a two-layer representatIon. In the shallow areas where the position of the interface between the upper (surface) and the lower (bottom) layers is equal to or lower than that of the bed the bottom layer vanishes and thus, the surface layer is only used, just as in a depth-averaged method (Cha~ and lin, 1995).
L Figure 3. Computational grid.
Solution of the hydrodynamic param~ters .has been calibrated ~reviousl~ (Chau an~ lin, 1995). It has been shown that the effects of density stratIfication are represented faIrly well In the solutIon. For the Tolo waters, flow directions in the surface and the bottom layer are inconsistent, especially when the current is not too swift. Figure 4 displays the velocity hodographs at three locations (~ee Figure 1) for a typical semidiumal tide. In side coves, the directions in the surface layer are generally dIfferent from those in the bottom layer (Chau and lin, 1995).
0.1
0.2
Vx (m/s)
Figure 4. Computed velocity hodographs in Tolo Harbour. Hong Kong.
230
H. JIN etal.
The definitions of layer-averaged water quality variables are the same as those of the hydrodynamic parameters (Chau and lin, 1995). For describing eutrophication. nine water quality variables are simulated. including three organic parameters (carbon represented by its equivalent CBOD. nitrogen and phosphorus). four inorganic parameters (dissolved oxygen. ammonia nitrogen. nitrite+nitrate nitrogen and orthophosphate) and two biological constituents (phytoplankton and zooplankton). NH 4• N0 2+N0 3 and P0 4 are considered as the nutrients available for phytoplankton uptake. The level of eutrophication due to excessive amounts of phytoplankton can be measured using several criteria. Although chlorophyll-a. that is an indicator of the gross level of phytoplankton does not provide information on species levels nor does it permit grouping into classes of phytoplankton. it is still the most common measure used in eutrophication studies and is adopted in the modeling. The structure of transport equations for water quality variables is referred to a generally accepted framework (Thomann and Mueller. 1987; Ambrose et al.• 1988) except the interaction between upper and lower layers and some kinetics parameters. The transport equations are in the form of a two-layered average. Herein. we introduce only the transport equations for phytoplankton and dissolved oxygen (see Chau and lin. 1998 for the details). A general form of the transport equation is:
where k=index of layer: k='u' for upper layer and 'I' for lower layer; subscripts "0". "s" and "b" denote the quantity at the layer interface. at free surface and at bed respectively;
=
FiiUfC 5. Phytoplankton and Dissolved Oxygen (DO) kinetics.
Carbon to chlorophyll-. ratio
231
S!p,1e represents the reaction kinetics, settling, sediment release, and external sources/sinkS ' th leth I Phytoplankton and DO kinetics are shown in Figure 5, Phytoplankton decreases due t Ithn e d - ayer. , , () I ked' , 0 e en ogenous ' respiration rA ' zoop an ton pr atlon or grazmg (C~), and nonpredatory mortality (M ) - II d 'a1 Ie, d' h' aI dama A ce th roug h bacten attac Isease, p YSlc ge, e natural ageing process or other m h estruction ' ' to th e sed'Iment (wSA ) or sm 'Ie (sA,k' ) It mcreases ' ec anlsms, settl mg due to its growth (J.1A k) and its direct ( and etc. The computations sho~ that zooplankton grazing plays an important role'in limiting a1galS~:':s~~~~ sources of DO are reaeratlon from the atmosphere (k.), photosynthetic oxygen production (J.1 ) d' source (soo k)' etc. Internal sinks of DO are: oxidation of carbonaceous waste material (k-\ oA.'dk'ti' lrecft , ' 'k d ' v' XI a on 0 nItrogenous waste matenal ( N), oxygen emand of sediments (SOD), use of oxygen for resp' f b aquatic plants (rA) and zooplankton (rz), etc, SOD is related to the oxidation of sediment algal c~~ Ion ~ other processes, Hence, S!p,1e for phytoplankton and DO kinetics can be expressed as follows: on an (3)
(jIAJ =h,(J.1AJ-rA-MA-C,Z,)AI-WsA(A,-A.)+SA.,
n
~
(4)
M
(jIoo,. =h'{t2[a"(~A" -rA)A. -rz Z.J+ 14 a\3(l-fpnr,.)A. -(lec BOO • +i4 kNNH •.• )
(5)
32 48 64 (jIooJ = hi {t2 [a"(~AJ -rA)A I - rzZI] + 14 a I3 (1- fpnrJ)A I -(kcBOOI + 14kNNH.J)}
(6)
+ k. (DO' - DO.)} + soo.•
-SOO+SooJ where Ak (J.1gChI-aII), Zk (mgZoopICII), NH4,k (J.1gNn), BOOk (mg 0 2/1) and OOk (mg0 2/1) are the layer• averaged concentrations of chl,orophyll-a, zooplan~ton, NH 4-N" CBOO and DO in kth-Iayer, respectively; Oos=OO saturation concentration (mg02/1); fj,ref.k IS an ammOnIa preference factor for algae uptake due to physiological reasons (Ambrose et ai., 1988). alJ is the stoichiometric ratio of cell nitrogen to algae chlorophyll (J.1gN/J.1gChl-a). al8 is the stoichiometric ratio of algae to organic carbon (mgC/mgChl-a), which is equal to the CCHL, The concentrations of all water quality variables at the open boundary are specified with measured data when water flows into the domain, and the normal gradients of the concentrations are simply taken as zero if water flows out from the waterbody, The normal gradients of the concentrations at bank boundary are specified as zero, The initial conditions are obtained according to the corresponding boundary values at starting time, The hydrodynamic parameters are calculated first. Then, the transport equation (1) is solved by control volume method (Patankar, 1980) in the orthogonal curvilinear mesh (Figure 3), As stated above, energy and mass balance for individual constituents is invariably linked to several others in the ecosystem. One of the key bridge~ is the stoichiometric ratio ,amon.g phytop,lan~ton chlorophyll-a, zooplankton carbon, organiC carbon, dlsso,lved oxygen, etc, EspeCially m 0,0 kInetiCS, photosynthetic oxygen production plays an important role via algal ~rowth, ,It me~s that the rat,lo ~f carbon to chlorophyll• a _ CCHL (mgClmgChI-a) is an important parameter 10 the Simulation of eutrophication, CARBON TO CHLOROPHYLL-A RATIO - CCHL The CCHL ratio is a variable, depending on the past hist~ry of algal cells, and is affected by light intensity, temperature, and nutrient availability, ~easured CCHL 10 laboratory cultures and natural populations v,ary widely, and can range in order of magnItude from 20, to 1000 (~e ,et ai" 1991), Phytoplankton may adjust its chlorophyll composition to adapt t~ the changes 10 solar radiation, If ~n,e adopts a constant saturating light intensity (I.) obtained by calibrating data co~lected under sun~y c~ndlti,ons, f~r ~xample, on overcast days excessively limited production may ~ p~edlcted" The saturatl~g bght ,lOtenslty IS the optimum light intensity at which the relative photosynthesIS IS a maximum, It vanes for different phytoplankton groups.
232
H. JIN etat.
Conversely, if Is is too small, on bright sunny days in summer, photoinhibition may be exaggerated. It is desirable to incorporate light acclimation by phytoplankton into the determination of Is and CCHL.
In situ measurements have often shown a maximum oxygen level at I-2m below the surface where the light
intensity is about 40-50% of that just underneath the surface, both in overcast and sunny conditions. Therefore, Is should be the weighted average of the light intensity for previous several days. Herein three days are chosen, i.e., , where I j=O.5x'i' days earlier daily average visible light intensity (photosynthetically active radiation which is about 43% of total incident light beneath the surface) (Lee et al., 1991). Is is assumed constant vertically for the algae in the water column. Furthermore, a variable carbon to chlorophyll• a ratio (CCHL), based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions of light and temperature, can be determined. The rate of photosynthesis (p) can be written as:
(7) where T is the water temperature,
e is the temperature correction factor, I is the light intensity at depth z.
J.l.maxA.20 is the maximum growth rate of phytoplankton at 20°C under optimal light and nutrient conditions.
It is a function of the species of phytoplankton, i.e., locality dependent, and can vary considerably. Herein, a constant J.l.max A.2o=2.l (day-I) is used for the Tolo waters. Initial slope of the p-I curve (a, mgClmgChl-aely) remains relatively unchanged in the extent of small I where lEal although maximum photosynthetic rates vary widely. At lower light intensities, the rate of photosynthesis depends only on the general photosynthetic reaction of chloroplast pigment and it should be independent of species and temperature (Ambrose et al., 1988; Lee et al., 1991). Bannister (Thomann and Mueller, 1987) gave good arguments for adopting 0.06mole carbon (O.07mole O2) per mole of photons as the maximum yield for plankton in nature. Based on these researches and in accordance with the extinction law of light with the water depth (Thomann and Mueller, 1987), an expression for the dynamic determination of CCHL for any given day can be derived as: (8)
In the simulation for Tolo Harbour, a=6.OmgClmgChl-aely from laboratory results (Lee et al., 1991) is used. APPLICATION OF THE MODEL TO TOLO HARBOUR, HONG KONG The annual average intensity of daily solar radiation in Tolo Harbour, Hong Kong, as shown in Figure 2(b), is about 300 Iy/day. In the Tolo waters' eutrophication simulation. an hourly light intensity
I:
~~~~8:19·~88~-'-~..l....I""""""""'~19l::89~-'-~..l....I 1989
1990
.....................I.-lI990
Time (year)
(a) In the surface layer
Time (year)
(b) In the bottom layer
Figure 6. Temporal CCHL in Tala Harbour during 1988-1989.
Carbon 10 chlorophyll-a ratio
233
Calculated temporal change of CCHL in Tolo Harbour during 1988-1989 is shown in Figure 6 for both the surface and the bottom layers. Generally, the CCHL varied around 50-250 mgClmgChl-a and was a bit higher in the ~ttom layer than that at the surface area, because water temperature in the bottom was lower than that in the surface. The averaged CCHLs at both the surface and the bottom layer during 1988-1989 were equal to 111.7 and 125.4 mgClmgChl-a, respectively. The CCHL in later summer and early autumn is generally lower because solar radiation gradually decreases and water temperature is still higher (Chau and Jin, 1995), which is the nature in subtropical zone. The corresponding chlorophyll-a and DO computed by the model with the above-mentioned temporal CCHL are listed in Figure 7. These results show that the predictions agree with the observations. The method described reasonably the stratification feature in Tolo water during summer, especially on serious oxygen depletion and even anoxic condition in the bottom area although the surface water may still be well oxygenated. Tolo water is well mixed in winter, and thus the stratification almost diminishes. Such nature corresponds to the density stratification in the water body as shown in Figure 2(a).
~ ------~-------- ~
~~-,----------~~--------------~-
I ____ ..1 __ ._____
i5~
~
(b) AI TM7
(a)AlTMl
_-.- ___ -tI _______ _ ____ ..1I _______ _ •
I
(a)AlTMl
-
'
--------1-------I
••
I
-----~--------
----~--~.--- I
(c) AI TMS
-------1-------I
- ____ ..1I _______ _ I
(c)AlTMS
1990 Time (year)
(2) In Ibe bottom layer
Figure 7. Chloropbyll-. and 00 computed and measured in Tolo Harbour (- computed•• measured)
uses constant CCHL instead of formula (8). The results of The compu ted Do may be quite different if one . h' h th th t d . d' th DO with CCHL= 111.7 are shown in Figure 8, ~n w IC eo. er ~arame ers are . etermme m e same way as for the above simulation. In comparison WIth the results m Figure 7, th~re IS a larger gap. between the results of DO computed with constant CCHL and the measurements. The s~nous oxygen depletion cannot be · th calculation with constant CCHL. Such facts emphasize that the CCHL evaluated by reproduced m e . ld be ak . . th . formula (8) is reasonable and the dynarruc CCHL shou t en IDto account m e long-term evaluation of eutrophication phenomenon.
234
H. JIN etal. ~~---------r------~
E~
80-
~~:~-------r------~
~ -------~-------80I
... (I) At TMl
(b) At TM7
(c) At TMS
(1) In the surface layer ~~-------r------~
L
800
1989
(a) At TMl
1990
Time (year)
----- --4-.- -----I
f9'~88~"""""""'"'+.':-!:!-"""""""''''''''~
t990
(c) At TMS
Time (year)
(2) In the bottom layer
Figure 8. DO computed with constant CCHL=111.7 and measured in Tolo Harbour (- computed•• measured)
CONCLUSIONS Algae may adjust their chlorophyll composition to adapt to the changes in ambient conditions, e.g.• solar radiation. water temperature, etc. Accordingly. carbon to chlorophyll-a ratio-CCHL is formulated in relation to solar radiation and water temperature. The calculated CCHL in Tolo Harbour, Hong Kong is generally around 50 - 250 mgClmgChl-a and is lower in the later summer and early autumn due to the subtropical climate. The Tolo water's eutrophication phenomenon and bottom oxygen depletion in the long time span have been represented well by a two-layered, two-dimensional numerical method in which density stratification is considered and the light acclimation by phytoplankton is incorporated into the evaluation of Is and CCHL. The calculation with a constant CCHL cannot represent the bottom oxygen depletion in the Tolo waters. These emphasize that it is reasonable to employ the optimum light intensity (IS> as weighted averaged solar radiation for previous three days and it is necessary to determine CCHL dynamically. ACKNOWLEDGEMENT The EPD, the Royal Observatory and the Drainage Service Department of Hong Kong are gratefully acknowledged for providing the monitoring data in Tolo Harbour, Hong Kong. REFERENCES Ambrose. R. B.• Wool. T. A.• Connolly. 1. P. and Schanz. R. W. (1988). WASP4. a hydrodynamic and wattr quality model· model theory. user's manual, and programmer's guide, Report EPA/6OOI3-87/039, u.S. Environmental Protection Agency, USA. Bothwell. M. L. (1992). Eutrophication of riven by nutrients in treated kraft pulp mill effluent. Water Pollution Research JoumIJl o/Canada. 27(3). 447-472. Cereo. C. F. and Cole. T. (1993). Three-dimensional eutrophication model of Chesapeake bay. JoumlJl of Environmental Engineering. ASCE.119(6). 1()()6.1025. Chau. K. W. and lin. H. S. (1995). Numerical solution of two-layered. two-dimensional tidal flow in boundary·fitted orthogonal curvilinear co-ordinate .ystem.rntemlJtioMI JOUmlJl/or Numerical Methods in Fluids. 21(11). 1087·1107. Chau. K. W. and lin. H. S. (1998). Eutrophication model for I coastal bay in Hong Kong. JoumlJl 0/ Environmental Engineering. ASCE. 124(7). 628-638. EPDHK (1990). Marine water quality in Hong Kong: results from the EPD marine water quality monitoring programme/or 1989. Report EPfTR 3/90. Environmental Protection Department, Hong Kong. Fischer. H. B.• List, E.I .• Koh. R. C. Y .• Imberger.I. and Brooks. N. H. (1979). Mixing in rnland and Coastal Watm. Academic Press. Inc .• Orlando. Florida. Lee. 1. H. W.• Wu. R. S. S.• C1eung. Y. K. and Wong. P. P. S. (1991). Forecasting of dissolved oxygen in marine fish culture zone. JoumlJl o/Environmental Engineering. ASCE.117(6). 816-833.
Carbon to chlorophyll-a ratio
23S
Patankar. S. V. (1980). Numerical Heal Transfer and Fluid Flow. Hemisphere Publishing Corporation. Rodi. W. (1980). Turbulence Models and Their Applicalion in Hydraulics. state-of-the-an. IAHR Publication. DELFI'. The Netherlands Salhotra. A. M.• Adams. E. E. and Harleman. D. R. F. (1987). Vertical mixing in thermohaline system. In: Proceedings of the Third International Symposium on Stratified Flows. EJohn List and Gerhard H.lirka (cds.). ASCE Publication. 10151026. Pasadena. California. Feb.3-S. 1987. SWEC (1996). To a favourable change of Biwa lake and Yodo river - A proposal. Chap.2 (40-119). Secretariat of Water Environmental Conference of Biwa Lake and Yodo River. Japan. Thomann. R. V. and Mueller. J. A. (1987). Principles of Surface Water Quality Modeling and Control. Harper and Row Publishers. New York. Van Eden. A. (1993). Marine pollution and the environment of the North Sea. European Water Pollution Control. 3(S). 16-24. Yih Chia-Shun (1980). Stratified Flows. Academic Press.
Pergamon
Wat. ScL Tech. Vol. 38. No. II. pp. 227-23.5.1998. ~ 1998IAWQ .
.
PIT: S0273-1223(98)00659-3
. Published by Elsevier Science LId
Printed In Great Britain. All rights reserved 0273-1223198 $19'00 + 0-00
CARBON TO CHLOROPHYLL-A RATIO IN MODELING LONG-TERM EUTROPHICATION PHENOMENA Haisheng lin *, Shinji Egashira** and K. W. Chau *** • R&D Center. Nippon Koei Co.• Ltd. 2304 Takasaki. Kukizaki-cho. lnashiki-gun • Ibaraki 300. Japan •• Faculty a/Science and Engineering. Ritsumeikan University. ]-]-] Noji-higashi. Kusatsu. Shiga 525. Japan ••• Department a/Civil and Structural Engineering. Hong Kong Polytechnic University. Hung Hom. Kowloon. Hong Kong
ABSTRACT Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur 50 as to max~ize the specifi.c growth rate for ambient conditions. including solar radiation and water temperature. With the dynamiC CCHL. an unsteady two-layered. two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (1s) is determined as weighted average of the light i.ntensit~ ~or previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxIc condition during summer in Tolo Harbour. Hong Kong is successfully reproduced by the present method. Otherwise. the simulation with a constant CCHL gave a wrong result.
KEYWORDS Carbon to chlorophyll-a ratio; density stratification; dissolved oxygen; eutrophication; phytoplankton; tidal flow.
INTRODUCTION Eutrophication. due to excessive grow~ of aquatic plants ~o levels resulting in the interference with desirable water uses. is one of the most Important water quality problems. The polluting material yielded from the extensive industrial West European basins of Rhine. Meuse and Scheidt has inevitably caused a serious pollution in the North Sea (Ede~ •. 19~3). Nutrie.nts in kraft pulp mill effluent have resulted in major changes in the level of algal produc~vlty In som~ nve~ of western North ~merica (Bothwell. 1992). Chesapeake Bay in USA is plagued WI~ probl~ms. including bottom-water. anoxIa, decline in fisheries and so on. that accompany agricultural and Industnal developme~t an~ population growth along its shores and headwaters (Cerco and Cole. 1993). Since 1977. algal bloom In Blwa lake of Japan has usually occurred in early summer (SWEC. 1996). In Hong Kong. most o~ the. coastal water~ recei~e sewage discharges directly from the urbanized catchments. as well as domestic. livestock and Industnal sources via streams and stormwater runoffs. which results in excessive nutrient contents with high eutrophic potential and development of algal blooms including red tides. In Tolo Harbour of Hong Kong. there were 89 red tide 227
228
H.lIN elal.
occurrences with 39 cases recorded in 1988. The monitored maximum chlorophyll-a concentrations were above 80mgll in 1986, 70mgll in 1988 and 60mgll in 1989, respectively (EPDHK, 1990). In general, mean chlorophyll-a levels above IOmgll can be regarded as unacceptably high and such levels indicate eutrophication. For natural offshore oceanic waters, the chlorophyll-a levels are below 2mgll. Tolo waters can be regarded as highly eutrophic. The problem has been attended seriously. Much effort and many countermeasures have been conducted in recovering the equilibrium of ecosystem (Thomann and Mueller, 1987; Ambrose et al., 1988; Lee et al., 1991; Cerco and Cole, 1993). With the aim of predicting long-term water quality transport and fate accompanied by eutrophication in a density stratified natural waterbody, a two-layered, two-dimensional eutrophication numerical model has been developed and calibrated by the scenario in Tolo Harbour of Hong Kong. The model is based on a numerical-generated boundary-fitted orthogonal curvilinear co-ordinate system. The hydrodynamic parameters are predicted simultaneously (Chau and lin, 1995). Pollution sources are locally specified. Referring to in situ sampling analysis, the sediment oxygen demand and nutrients released from sediment are incorporated in the modeling. In this paper, an important parameter CCHL Carbon to CHLorophyll-a ratio is emphasized and corresponding results are discussed. BRIEF DESCRIPTION OF EUTROPHICATION MODELING
In a water quality system, density stratification usually forms, which inhibits vertical transport and suppresses the connection between benthic grazers and near-surface biomass (Salhotra et al., 1987). Despite dissolved oxygen content in most of the surface waters are at satisfactory levels, even at super-saturation, serious oxygen depletion, approaching anoxic conditions may occur in the lower water (Cerco and Cole, 1993).
Tolo Harbour is a nearly land-locked sea inlet with water surface area of about 52 km 2 as shown in Figure I. There are approximate 16km long from southwest at inner harbour to northeast at outer channel. The water depth is from about 2m in the inner part to over 20m in the outer and about 12m on average. For most of the year, little freshwater flows into the harbour. During summer, the differences of temperature as well as salinity in surface and bottom water results in density stratification, which exhibits an obvious lighter surface layer and a definite mesolimnion. In winter, the stratification almost diminishes. Multi-yearly-averaged daily density variation in both the surface and the bottom water is shown in Figure 2(a). It is much better to describe the transport and fate of water quality with a two-layer-averaged system in the vertical water column instead of a common depth-averaged method (Yih Chia-Shun, 1980). According to the measured data, the layer interface position is located at Zo=-6m (M.C.D. - Metre Chart Datum) for the Tolo waters. Tolo Harbour is located in subtropical zone. Figure 2(b) shows its daily solar radiation intensity during 1985-1989. The New Territories, Hong Kong KauLauWan
o
• monitoring station
I 2km
C'"n-e-H-On-g-K-O-ns-u-I-an-d-)
Figure 1. Tolo Harbour, Hong Kong.
Pollution sources: - main stream a outfall
Carbon 10 chlorophyll-a ratio
229
(a) Water density Figure 2. Water density and daily solar radiation in Tolo Harbour. Hong Kong.
Due to irregular computational domain, a numerically generated boundary-fitted orthogonal curvilinear co• ordinate system, as shown in Figure 3, is employed. In orthogonal curvilinear coordinates, the present method numerically solves partial differential equations describing conservation of mass and momentum of incompressible fluid as well as transport of po.llutants over the depth of each layer or over the total depth in areas too shallow for a two-layer representatIon. In the shallow areas where the position of the interface between the upper (surface) and the lower (bottom) layers is equal to or lower than that of the bed the bottom layer vanishes and thus, the surface layer is only used, just as in a depth-averaged method (Cha~ and lin, 1995).
L Figure 3. Computational grid.
Solution of the hydrodynamic param~ters .has been calibrated ~reviousl~ (Chau an~ lin, 1995). It has been shown that the effects of density stratIfication are represented faIrly well In the solutIon. For the Tolo waters, flow directions in the surface and the bottom layer are inconsistent, especially when the current is not too swift. Figure 4 displays the velocity hodographs at three locations (~ee Figure 1) for a typical semidiumal tide. In side coves, the directions in the surface layer are generally dIfferent from those in the bottom layer (Chau and lin, 1995).
0.1
0.2
Vx (m/s)
Figure 4. Computed velocity hodographs in Tolo Harbour. Hong Kong.
230
H. JIN etal.
The definitions of layer-averaged water quality variables are the same as those of the hydrodynamic parameters (Chau and lin, 1995). For describing eutrophication. nine water quality variables are simulated. including three organic parameters (carbon represented by its equivalent CBOD. nitrogen and phosphorus). four inorganic parameters (dissolved oxygen. ammonia nitrogen. nitrite+nitrate nitrogen and orthophosphate) and two biological constituents (phytoplankton and zooplankton). NH 4• N0 2+N0 3 and P0 4 are considered as the nutrients available for phytoplankton uptake. The level of eutrophication due to excessive amounts of phytoplankton can be measured using several criteria. Although chlorophyll-a. that is an indicator of the gross level of phytoplankton does not provide information on species levels nor does it permit grouping into classes of phytoplankton. it is still the most common measure used in eutrophication studies and is adopted in the modeling. The structure of transport equations for water quality variables is referred to a generally accepted framework (Thomann and Mueller. 1987; Ambrose et al.• 1988) except the interaction between upper and lower layers and some kinetics parameters. The transport equations are in the form of a two-layered average. Herein. we introduce only the transport equations for phytoplankton and dissolved oxygen (see Chau and lin. 1998 for the details). A general form of the transport equation is:
where k=index of layer: k='u' for upper layer and 'I' for lower layer; subscripts "0". "s" and "b" denote the quantity at the layer interface. at free surface and at bed respectively;
=
FiiUfC 5. Phytoplankton and Dissolved Oxygen (DO) kinetics.
Carbon to chlorophyll-. ratio
231
S!p,1e represents the reaction kinetics, settling, sediment release, and external sources/sinkS ' th leth I Phytoplankton and DO kinetics are shown in Figure 5, Phytoplankton decreases due t Ithn e d - ayer. , , () I ked' , 0 e en ogenous ' respiration rA ' zoop an ton pr atlon or grazmg (C~), and nonpredatory mortality (M ) - II d 'a1 Ie, d' h' aI dama A ce th roug h bacten attac Isease, p YSlc ge, e natural ageing process or other m h estruction ' ' to th e sed'Iment (wSA ) or sm 'Ie (sA,k' ) It mcreases ' ec anlsms, settl mg due to its growth (J.1A k) and its direct ( and etc. The computations sho~ that zooplankton grazing plays an important role'in limiting a1galS~:':s~~~~ sources of DO are reaeratlon from the atmosphere (k.), photosynthetic oxygen production (J.1 ) d' source (soo k)' etc. Internal sinks of DO are: oxidation of carbonaceous waste material (k-\ oA.'dk'ti' lrecft , ' 'k d ' v' XI a on 0 nItrogenous waste matenal ( N), oxygen emand of sediments (SOD), use of oxygen for resp' f b aquatic plants (rA) and zooplankton (rz), etc, SOD is related to the oxidation of sediment algal c~~ Ion ~ other processes, Hence, S!p,1e for phytoplankton and DO kinetics can be expressed as follows: on an (3)
(jIAJ =h,(J.1AJ-rA-MA-C,Z,)AI-WsA(A,-A.)+SA.,
n
~
(4)
M
(jIoo,. =h'{t2[a"(~A" -rA)A. -rz Z.J+ 14 a\3(l-fpnr,.)A. -(lec BOO • +i4 kNNH •.• )
(5)
32 48 64 (jIooJ = hi {t2 [a"(~AJ -rA)A I - rzZI] + 14 a I3 (1- fpnrJ)A I -(kcBOOI + 14kNNH.J)}
(6)
+ k. (DO' - DO.)} + soo.•
-SOO+SooJ where Ak (J.1gChI-aII), Zk (mgZoopICII), NH4,k (J.1gNn), BOOk (mg 0 2/1) and OOk (mg0 2/1) are the layer• averaged concentrations of chl,orophyll-a, zooplan~ton, NH 4-N" CBOO and DO in kth-Iayer, respectively; Oos=OO saturation concentration (mg02/1); fj,ref.k IS an ammOnIa preference factor for algae uptake due to physiological reasons (Ambrose et ai., 1988). alJ is the stoichiometric ratio of cell nitrogen to algae chlorophyll (J.1gN/J.1gChl-a). al8 is the stoichiometric ratio of algae to organic carbon (mgC/mgChl-a), which is equal to the CCHL, The concentrations of all water quality variables at the open boundary are specified with measured data when water flows into the domain, and the normal gradients of the concentrations are simply taken as zero if water flows out from the waterbody, The normal gradients of the concentrations at bank boundary are specified as zero, The initial conditions are obtained according to the corresponding boundary values at starting time, The hydrodynamic parameters are calculated first. Then, the transport equation (1) is solved by control volume method (Patankar, 1980) in the orthogonal curvilinear mesh (Figure 3), As stated above, energy and mass balance for individual constituents is invariably linked to several others in the ecosystem. One of the key bridge~ is the stoichiometric ratio ,amon.g phytop,lan~ton chlorophyll-a, zooplankton carbon, organiC carbon, dlsso,lved oxygen, etc, EspeCially m 0,0 kInetiCS, photosynthetic oxygen production plays an important role via algal ~rowth, ,It me~s that the rat,lo ~f carbon to chlorophyll• a _ CCHL (mgClmgChI-a) is an important parameter 10 the Simulation of eutrophication, CARBON TO CHLOROPHYLL-A RATIO - CCHL The CCHL ratio is a variable, depending on the past hist~ry of algal cells, and is affected by light intensity, temperature, and nutrient availability, ~easured CCHL 10 laboratory cultures and natural populations v,ary widely, and can range in order of magnItude from 20, to 1000 (~e ,et ai" 1991), Phytoplankton may adjust its chlorophyll composition to adapt t~ the changes 10 solar radiation, If ~n,e adopts a constant saturating light intensity (I.) obtained by calibrating data co~lected under sun~y c~ndlti,ons, f~r ~xample, on overcast days excessively limited production may ~ p~edlcted" The saturatl~g bght ,lOtenslty IS the optimum light intensity at which the relative photosynthesIS IS a maximum, It vanes for different phytoplankton groups.
232
H. JIN etat.
Conversely, if Is is too small, on bright sunny days in summer, photoinhibition may be exaggerated. It is desirable to incorporate light acclimation by phytoplankton into the determination of Is and CCHL.
In situ measurements have often shown a maximum oxygen level at I-2m below the surface where the light
intensity is about 40-50% of that just underneath the surface, both in overcast and sunny conditions. Therefore, Is should be the weighted average of the light intensity for previous several days. Herein three days are chosen, i.e., , where I j=O.5x'i' days earlier daily average visible light intensity (photosynthetically active radiation which is about 43% of total incident light beneath the surface) (Lee et al., 1991). Is is assumed constant vertically for the algae in the water column. Furthermore, a variable carbon to chlorophyll• a ratio (CCHL), based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions of light and temperature, can be determined. The rate of photosynthesis (p) can be written as:
(7) where T is the water temperature,
e is the temperature correction factor, I is the light intensity at depth z.
J.l.maxA.20 is the maximum growth rate of phytoplankton at 20°C under optimal light and nutrient conditions.
It is a function of the species of phytoplankton, i.e., locality dependent, and can vary considerably. Herein, a constant J.l.max A.2o=2.l (day-I) is used for the Tolo waters. Initial slope of the p-I curve (a, mgClmgChl-aely) remains relatively unchanged in the extent of small I where lEal although maximum photosynthetic rates vary widely. At lower light intensities, the rate of photosynthesis depends only on the general photosynthetic reaction of chloroplast pigment and it should be independent of species and temperature (Ambrose et al., 1988; Lee et al., 1991). Bannister (Thomann and Mueller, 1987) gave good arguments for adopting 0.06mole carbon (O.07mole O2) per mole of photons as the maximum yield for plankton in nature. Based on these researches and in accordance with the extinction law of light with the water depth (Thomann and Mueller, 1987), an expression for the dynamic determination of CCHL for any given day can be derived as: (8)
In the simulation for Tolo Harbour, a=6.OmgClmgChl-aely from laboratory results (Lee et al., 1991) is used. APPLICATION OF THE MODEL TO TOLO HARBOUR, HONG KONG The annual average intensity of daily solar radiation in Tolo Harbour, Hong Kong, as shown in Figure 2(b), is about 300 Iy/day. In the Tolo waters' eutrophication simulation. an hourly light intensity
I:
~~~~8:19·~88~-'-~..l....I""""""""'~19l::89~-'-~..l....I 1989
1990
.....................I.-lI990
Time (year)
(a) In the surface layer
Time (year)
(b) In the bottom layer
Figure 6. Temporal CCHL in Tala Harbour during 1988-1989.
Carbon 10 chlorophyll-a ratio
233
Calculated temporal change of CCHL in Tolo Harbour during 1988-1989 is shown in Figure 6 for both the surface and the bottom layers. Generally, the CCHL varied around 50-250 mgClmgChl-a and was a bit higher in the ~ttom layer than that at the surface area, because water temperature in the bottom was lower than that in the surface. The averaged CCHLs at both the surface and the bottom layer during 1988-1989 were equal to 111.7 and 125.4 mgClmgChl-a, respectively. The CCHL in later summer and early autumn is generally lower because solar radiation gradually decreases and water temperature is still higher (Chau and Jin, 1995), which is the nature in subtropical zone. The corresponding chlorophyll-a and DO computed by the model with the above-mentioned temporal CCHL are listed in Figure 7. These results show that the predictions agree with the observations. The method described reasonably the stratification feature in Tolo water during summer, especially on serious oxygen depletion and even anoxic condition in the bottom area although the surface water may still be well oxygenated. Tolo water is well mixed in winter, and thus the stratification almost diminishes. Such nature corresponds to the density stratification in the water body as shown in Figure 2(a).
~ ------~-------- ~
~~-,----------~~--------------~-
I ____ ..1 __ ._____
i5~
~
(b) AI TM7
(a)AlTMl
_-.- ___ -tI _______ _ ____ ..1I _______ _ •
I
(a)AlTMl
-
'
--------1-------I
••
I
-----~--------
----~--~.--- I
(c) AI TMS
-------1-------I
- ____ ..1I _______ _ I
(c)AlTMS
1990 Time (year)
(2) In Ibe bottom layer
Figure 7. Chloropbyll-. and 00 computed and measured in Tolo Harbour (- computed•• measured)
uses constant CCHL instead of formula (8). The results of The compu ted Do may be quite different if one . h' h th th t d . d' th DO with CCHL= 111.7 are shown in Figure 8, ~n w IC eo. er ~arame ers are . etermme m e same way as for the above simulation. In comparison WIth the results m Figure 7, th~re IS a larger gap. between the results of DO computed with constant CCHL and the measurements. The s~nous oxygen depletion cannot be · th calculation with constant CCHL. Such facts emphasize that the CCHL evaluated by reproduced m e . ld be ak . . th . formula (8) is reasonable and the dynarruc CCHL shou t en IDto account m e long-term evaluation of eutrophication phenomenon.
234
H. JIN etal. ~~---------r------~
E~
80-
~~:~-------r------~
~ -------~-------80I
... (I) At TMl
(b) At TM7
(c) At TMS
(1) In the surface layer ~~-------r------~
L
800
1989
(a) At TMl
1990
Time (year)
----- --4-.- -----I
f9'~88~"""""""'"'+.':-!:!-"""""""''''''''~
t990
(c) At TMS
Time (year)
(2) In the bottom layer
Figure 8. DO computed with constant CCHL=111.7 and measured in Tolo Harbour (- computed•• measured)
CONCLUSIONS Algae may adjust their chlorophyll composition to adapt to the changes in ambient conditions, e.g.• solar radiation. water temperature, etc. Accordingly. carbon to chlorophyll-a ratio-CCHL is formulated in relation to solar radiation and water temperature. The calculated CCHL in Tolo Harbour, Hong Kong is generally around 50 - 250 mgClmgChl-a and is lower in the later summer and early autumn due to the subtropical climate. The Tolo water's eutrophication phenomenon and bottom oxygen depletion in the long time span have been represented well by a two-layered, two-dimensional numerical method in which density stratification is considered and the light acclimation by phytoplankton is incorporated into the evaluation of Is and CCHL. The calculation with a constant CCHL cannot represent the bottom oxygen depletion in the Tolo waters. These emphasize that it is reasonable to employ the optimum light intensity (IS> as weighted averaged solar radiation for previous three days and it is necessary to determine CCHL dynamically. ACKNOWLEDGEMENT The EPD, the Royal Observatory and the Drainage Service Department of Hong Kong are gratefully acknowledged for providing the monitoring data in Tolo Harbour, Hong Kong. REFERENCES Ambrose. R. B.• Wool. T. A.• Connolly. 1. P. and Schanz. R. W. (1988). WASP4. a hydrodynamic and wattr quality model· model theory. user's manual, and programmer's guide, Report EPA/6OOI3-87/039, u.S. Environmental Protection Agency, USA. Bothwell. M. L. (1992). Eutrophication of riven by nutrients in treated kraft pulp mill effluent. Water Pollution Research JoumIJl o/Canada. 27(3). 447-472. Cereo. C. F. and Cole. T. (1993). Three-dimensional eutrophication model of Chesapeake bay. JoumlJl of Environmental Engineering. ASCE.119(6). 1()()6.1025. Chau. K. W. and lin. H. S. (1995). Numerical solution of two-layered. two-dimensional tidal flow in boundary·fitted orthogonal curvilinear co-ordinate .ystem.rntemlJtioMI JOUmlJl/or Numerical Methods in Fluids. 21(11). 1087·1107. Chau. K. W. and lin. H. S. (1998). Eutrophication model for I coastal bay in Hong Kong. JoumlJl 0/ Environmental Engineering. ASCE. 124(7). 628-638. EPDHK (1990). Marine water quality in Hong Kong: results from the EPD marine water quality monitoring programme/or 1989. Report EPfTR 3/90. Environmental Protection Department, Hong Kong. Fischer. H. B.• List, E.I .• Koh. R. C. Y .• Imberger.I. and Brooks. N. H. (1979). Mixing in rnland and Coastal Watm. Academic Press. Inc .• Orlando. Florida. Lee. 1. H. W.• Wu. R. S. S.• C1eung. Y. K. and Wong. P. P. S. (1991). Forecasting of dissolved oxygen in marine fish culture zone. JoumlJl o/Environmental Engineering. ASCE.117(6). 816-833.
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23S
Patankar. S. V. (1980). Numerical Heal Transfer and Fluid Flow. Hemisphere Publishing Corporation. Rodi. W. (1980). Turbulence Models and Their Applicalion in Hydraulics. state-of-the-an. IAHR Publication. DELFI'. The Netherlands Salhotra. A. M.• Adams. E. E. and Harleman. D. R. F. (1987). Vertical mixing in thermohaline system. In: Proceedings of the Third International Symposium on Stratified Flows. EJohn List and Gerhard H.lirka (cds.). ASCE Publication. 10151026. Pasadena. California. Feb.3-S. 1987. SWEC (1996). To a favourable change of Biwa lake and Yodo river - A proposal. Chap.2 (40-119). Secretariat of Water Environmental Conference of Biwa Lake and Yodo River. Japan. Thomann. R. V. and Mueller. J. A. (1987). Principles of Surface Water Quality Modeling and Control. Harper and Row Publishers. New York. Van Eden. A. (1993). Marine pollution and the environment of the North Sea. European Water Pollution Control. 3(S). 16-24. Yih Chia-Shun (1980). Stratified Flows. Academic Press.